1. Field of the Invention
The present invention relates to a method of producing a wired circuit board. More particularly, the present invention relates to a method of producing a wired circuit board such as a suspension board with circuit.
2. Description of the Related Art
A hard disk drive is equipped with a suspension board with circuit on which a magnetic head is mounted. In a typical suspension board with circuit, an insulating layer made of a resin and a conductive pattern made of copper are formed in this order on a metal supporting board made of stainless steel.
In such a suspension board with circuit, since a metal supporting board is formed of stainless steel, a transmission loss in a conductive pattern is increased.
To reduce the transmission loss, it is proposed that an insulating layer is formed on a suspension made of stainless steel, a lower conductor made of copper or a copper alloy is formed on the insulating layer, and an insulating layer, and conductors of a record side and a reproduction side are formed in this order on the lower conductor (see, e.g., Japanese Unexamined Patent No. 2005-11387).
A suspension board with circuit is also proposed which includes a metal supporting board, a metal thin film formed on the metal supporting board, a metal foil formed on the metal thin film, an insulating base layer formed on the metal foil, and a conductive pattern formed on the insulating base layer (see, e.g., Japanese Unexamined Patent No. 2006-245220).
The suspension board with circuit is produced as follows. That is, a metal supporting board is prepared first, and a metal thin film is formed on an entire surface of the metal supporting board by sputtering. Then, a resist is formed on the surface of the metal thin film in a pattern reverse to the pattern of a metal foil. Then, using the resist as a plating resist, the metal foil is formed on the surface of the metal thin film exposed from the resist by electrolytic plating. Thereafter, the resist and the portion of the metal thin film where the resist is formed are removed. Subsequently, a solution (varnish) of a synthetic resin is coated on the respective surfaces of the metal foil and the metal supporting board, dried, and cured to form the insulating base layer. Then, the conductive pattern is formed on the surface of the insulating base layer.